I'm not entirely sure what the criteria are that need to be met before a ninja is considered to be of S class. It looks to be some sort of combination of speed, strength, jutsu, etc. Does one additionally need some special techniques or bloodlines? Like Mangekyo Sharingan, Mokuton, Sage Mode, high class summons and/or kinjutsu? Are jonin like Kakashi Hatake and Might Guy S ranked? If so, at what point would you say they show the skills to be acknowledged as such?
The ranking classification is mainly used for techniques and missions. For eg: "X is an A-rank technique" or "Y is a B-rank mission".
The ranks determine the difficulty or level of danger involved in the mission or technique mastering/usage.
Out of the six ranks, only the S-Rank is used to classify a shinobi's class.
S-rank is usually used to classify techniques intended for highly experienced jōnin and Kage-level shinobi. S-rank techniques are almost always unique to a single user, and as such are trademark abilities of that user.
Normally, S-rank isn't titled to Jōnin or Kage level ninjas.
"S-rank" is also a designation given to extremely powerful missing-nin, earning them a place in their village of origin's Bingo Book.
S-rank is used to classify only the most dangerous and powerful missing nins because of their level of skill, exceptional skill set and usage of (ultra) rare techniques.
So, for a shinobi to be classified as "S-Rank", they must have a unique and very powerful skill set and most importantly, they must become a missing-nin.
source: Naruto Wikia